The present invention relates to a pneumatic tire, and more particularly a rib pattern radial ply tire, which is provided in both side portions of the tread with a plurality of sipes to prevent the partial wear of the tread.
In general, a tread part of a radial ply tire is reinforced with a belt layer which comprises plural plies of steel cords arranged at an angle of 15.degree. to 70.degree. with respect to the circumferential ddirection of the tire. Accordingly, the tread part of such radial tire is rigid, and both side portions of the tread becomes smaller in ground pressure than the central portion. As a result, the amount of slip of the tread on the ground becomes larger in the side portions than the central portion, and uneven wear in the tread shoulders, what is called shoulder wear is most liable to happen. This problem is particularly noticeaable for heavy duty tires for trucks and buses.
In order to reduce the problems of such uneven wear, it is known in the art to provide plural sipes in the both side portions of the tread lending to lessen the slip of those portions on the ground by means of the lowered apparent rigidity of the tread rubber which makes it possible to stay on the ground.
Such a construction is shown in FIG. 10, in which the sipes (S) are shallow and arranged parallel to the axial direction of the tire, and their lengths SW are less than 5% of the tread width, and further the circumferential grooves which define the side ribs are zigzag. Such sipes (S) are however, so short and shallow that they cannot control the distribution of the ground pressure in the widthwise direction of the tread, that is, they are not effective in preventing the uneven wear.
Also in Japanese Gazette TOKKAI No. 58-162643, as shown in FIG. 11, the sipes are provided on the blocks SB which are defined by a zigzag circumferential groove SG and axial grooves extending therefrom the to tread edge. Essentially such free blocks are liable to move. Therefore the sipes decrease the rigidity still more specifically in the projecting portion of the zigzag edge of the block, and uneven wear so called railway wear is caused in early stage.
It is therefore, an object of the present invention to provide a radial ply tire especially suitable for heavy duty use, in which the problems of uneven wear is reduced by unifying the amount of slip of the tread on the ground in the widthwise direction thereof.